Despite some characters' efforts, love is not logical, predictable, or even controllable. In Mansfield Park, characters constantly fall in love with people they'd rather not love. They fall for people who don't love them back, they try to love people whom they can't, and they try to fall out of love with people and fail. Love is very messy and very confusing here. It challenges characters' attitudes and preconceptions. But there is an exception to all this drama. Fanny experiences love as practically predictable. Her love for Edmund doesn't make her change; rather it reaffirms who she is. It's questionable as to whether or not love is preferable at its messiest or at its safest.
Edmund's romance with Fanny is inevitable given Fanny's status as a protagonist; she has to get what she wants in the end.
If Fanny had taken the chance, she actually could have fallen in love with Henry and could have been happy with him.